This is a single-blind, randomized, and controlled trial of an innovative approach to fall prevention in individuals with Parkinson's disease. We hypothesize that gait and step perturbation (GSP) will increase gait speed, cadence, stride length, dynamic balance, and reduce fall frequency and Gait and Balance Scores compared to a group who receives seated exercise (SPT). Inclusion criteria: 1) diagnosis of PD;2) postural instability- gait disorder predominant PD;3) history of falls, gait freezing or a positive pull test;4) stable regimen of medications;5) ability to stand and walk 3-m without assistance;6) stage 2 or 3 of the Hoehn and Yahr disability scale;and 7) moderate or higher cognitive score. Ninety subjects will be randomly assigned to 1) GSP;or 2) SPT. GSP will receive gait training on a treadmill and in a safety harness while walking in 4 directions: frontwards, backwards, left sideways and right sideways. Training will start with a treadmill speed that is equivalent to fastest overground walking speed for forward walking, and fastest possible for other directions, and will increase during training. The GSP group will also receive step training while positioned in 4 directions consisting of suddenly turning the treadmill on /off. The subjects will be required to maintain their balance during perturbations. The SPT group will receive seated active range of motion, and upper and lower extremity aerobic training. The interventions will occur 1 hour/day, 3 times per week for 8 weeks. Tests will include usual and fastest gait speed, 5-step test, Unified Parkinson's Disease Rating Scale, Gait and Balance Scale, Center for Epidemiological Studies-Depression Scale, Cognistat, a co-morbidity scale, Activities Balance Confidence, Physical Activity Scale for the Elderly, and limits of stability. Planned comparisons will be made on the temporal and spatial parameters of gait (on and off medications), and dynamic balance (5-step test) {on and off medications} before and after an 8-week period and during 1- &5- month follow ups. Fall frequencies will be established for 5 months prior to and 5 months after the interventions.